If two persons with 'AB' blood group marry and have sufficiently large number of  children, these children could be classified as 'A' blood group: 'AB'  blood  group : 'B' blood group in 1 : 2 : 1 ratio. Modern technique of protein electrophoresis reveals presence of both 'A' and 'B' type proteins in 'AB' blood group individuals. This in an example of

In the case of co-dominance the F1 generation resembles both parents. A good example is different types of red blood cells that determine ABO blood grouping in human beings. ABO blood groups are controlled by the gene (I). This gene (I)has three alleles IA, IBand i. The alleles IAand IBproduce a slightly different form of the sugar while allele i does not produce any sugar.When IAand IBare present together they both express their own types of sugars: this is because of co-dominance. Hence red blood cells have both A and B types of sugars.